Python 'For' Loop Iterate Array

Question

I am new to python and I am looking for some help with loop structures, specifically how to use a 'For' loop or alternative loop to solve a problem. I need to figure a discounted price based on the previous days' discounted price in an array so it will iterate through the array an apply the 10% discount. Here is what I have so far:

opnDays = ["mon", "tue", "wed", "thr", "fri"]
price = 10
def discount(array):
 for day in array:
 disPrice = price - (price * .1)
 print(day, disPrice)
discount(opnDays)
>>>
mon 9.0
tue 9.0
wed 9.0
thr 9.0
fri 9.0

I want to get:

>>>
mon 9.0
tue 8.1
wed 7.29
thr 6.561
fri 5.9109

Any help with how to organize this loop to get a the 10% discount from the previous day would be helpful. Thank you

• Joran Beasley: price = price - (price * .1);print(day, price)
• PM 2Ring: "I need to figure a discounted price based on the previous days' discounted price in an array". Ok, but that code just uses an array (actually, it's a list) for the day names, but it sounds like you're also supposed to use one for the discounted prices as well. As the answers below show, you don't actually need that, but maybe the point of this exercise is to create a list that contains the discounted price for each day.
• Sheldore: @JoranBeasley: Your answer will work if either price is defined inside the function OR price is passed to the function in a variable also called price OR you will have to define global price inside the function.
• Joran Beasley: @Bazingaa :) thanks for the clarification :)
• PM 2Ring: Why on earth was this downvoted? Jay asked a clear & simple question, and showed us his code, along with expected output and actual output.

Answer 1

Your code, as written, is close:

opnDays = ["mon", "tue", "wed", "thr", "fri"]
price = 10
def discount(array):
 disPrice = price
 for day in array:
 disPrice *= 0.9
 print(day, disPrice)

What I did here was change how disPrice was set on your loop. You were setting it to the same value (0.9 * price) on every iteration. All I did was set it to price in the beginning and multiply it by 0.9 every iteration, resulting in the desired behavior.

• Jay: All very helpful beta. Appreciate the insight.

Answer 2

You're not changing the value of price so the same result is calculated on every iteration of the loop.

This should work:

for day in array:
 price = price - (price * .1)
 print(day, price)

• T Burgis: It would be better to calculate the price using price = price * 0.9
• Sheldore: Gives UnboundLocalError: local variable 'price' referenced before assignment. If I use global price inside the function and before the for loop as one of the answers below, it works
• Sheldore: @nosklo: I know, it is to replace the OP's for loop. But unless I define global price inside the function, I am getting this UnboundLocalError.

Answer 3

If you want to stick to using a function as in your code, following is one way:

opnDays = ["mon", "tue", "wed", "thr", "fri"]
price = 10
def discount(array):
 disPrice = price # assign initial price to discount
 for day in array:
 disPrice = disPrice*0.9 # Reduce the discount by 10 percent every time
 print(day, disPrice)
discount(opnDays)

Output

mon 9.0
tue 8.1
wed 7.29
thr 6.561
fri 5.9049000000000005

Answer 4

Your code is pretty close, but on each day you need to modify the price of the previous day.

I would pass the starting price into the function, just like you pass the list of day names. I've also used .format, so we get the prices printed as dollars & cents. This only affects how the price is printed, it doesn't affect the actual value, so we don't lose any accuracy.

I've also changed your names slightly so that they conform with the usual Python PEP-0008 style.

def discount(array, price):
 for day in array:
 price = price - (price * .1)
 print("{}: {:.2f}".format(day, price))
open_days = ["mon", "tue", "wed", "thr", "fri"]
start_price = 10.00
discount(open_days, start_price)

output

mon: 9.00
tue: 8.10
wed: 7.29
thr: 6.56
fri: 5.90

Answer 5

You can use global and keep your logic as it is

opnDays = ["mon", "tue", "wed", "thr", "fri"]
price = 10
def discount(array):
 global price
 for day in array:
 price = price - (price * .1)
 print(day, price)
discount(opnDays)

In order to avoid global pass the price as an argument and the preferred approach

opnDays = ["mon", "tue", "wed", "thr", "fri"]
price = 10
def discount(array,price):
 for day in array:
 price = price - (price * .1)
 print(day, round(price,2))
discount(opnDays,price)

In case you need the final price after successive iterations which I believe might be a requirement for you

opnDays = ["mon", "tue", "wed", "thr", "fri"]
price = 10
def discount(array,price):
 for day in array:
 price = price - (price * .1)
 print(day, round(price,2))
 return price
final_price = discount(opnDays,price)
print(final_price)

• PM 2Ring: You could use global for this, but it shouldn't be encouraged. Modifiable globals break program modularity. It doesn't matter much for a tiny program like this, but it's a bad habit to get into.
• mad_: I am usually not a fan of using global but in this case, I believe the final price should be the result of iterations in the sense that it will reduce 7 times as original price and should be able to consume later.
• Sheldore: @mad_: If I try the code without global price, I get UnboundLocalError: local variable 'price' referenced before assignment
• mad_: @Bazingaa obviously price has not been declared inside the funtion
• Sheldore: Yeah, so the solution where you just use price = price - (price * .1) without passing the price to the function would not work. That's what I am saying